At least three people had a hand writing this: original director Robert Hamer adjusted Donald Taylor's screenplay, which was based on C. E. Vulliamy's 1955 novel, "Don Among the Dead Men." Did someone along the way excise the cutting edge? Or was it never there to begin with?
A university don toying with toxins in his chem lab discovers a traceless poison and starts bumping off people who annoy him: the smalltown gossip (Patricia Jessel), a rival don, then another (Dennis Price), and his too clever by half mistress (Janet Munro). The tone is comic, and if you accept the smug, married, adulterous sociopath as anti-hero, you might still wonder whether to laugh or cry.
Leo McKern in bow tie with furled umbrella, or amongst the bunson burners, offers a comic peekaboo at the fiddlings of a biochemical research process. He makes white rats dance to his alchemical tune and fall comatose, given the right dosage... then, heigho, it's on to anyone who thwarts his ambitions.
The model of a modern amoral biochemist, the serial poisoner is a resonant cautionary tale, but he never achieves full-blown comic mania. An industrial scale might be required to incite an audience to sit up and howl. Alec Guinness in The Man in the White Suit (1951) had a larger budget.
The depiction of common room, pub, seaside getaway, high street flower shop, et al, is charming. There's a generous smattering of capable character actors not given enough to do. An exception is Patricia Jessel, whose busybody lush in denial is the only fully developed cameo. In fact, it's the women - Jesell, Munro, and Maxine Audley as the wife, who move things along. The print is bad but clear enough.